1. Technical Field
The invention relates to a method and a device for the measurement of ions by coupling different measurement methods/techniques.
2. Prior Art
The measurement of ions is important particularly in connection with mass-spectrometric analysis methods. In the scope of materials analysis, for instance, ions are generated from a material sample, separated according to mass or other criteria and detected in a detector or a similar instrument.
Collectors, for example Faraday cups, are widely known detectors which can be used to measure the ion current as a voltage across a high resistance or in a high impedance amplifier. Secondary electron multipliers (SEMs) are also known. They operate with a conversion dynode at the input, on whose surface the incoming ions are neutralized and electrons are thereupon released. The electrons are then multiplied from stage to stage inside the SEM, so that even very small numbers of ions can be registered. It is also already known to operate an SEM in two different operating modes, namely analog mode and count mode. In order to record the electrons in analog mode, a signal is taken from one of the central stages. The count mode records the electrons arriving at the last stage of the SEM. The analog mode and count mode run in parallel with each other, for instance in the Finnigan Element 2 mass spectrometer from Thermo Electron. High ion currents can be measured using the analog mode, while the count mode evaluates the relatively smaller ion currents.
In particular applications, it is expedient to have a wide dynamic measurement range of more than nine orders of magnitude (more than 109). In order to quantify minor impurities or doping in mass-spectrometric materials analysis, for example, such as laser ablation ICP mass spectrometry or glow discharge mass spectrometry (GD-MS), it is advantageous to be able to measure both the primary component (matrix) and the impurities or doping. It is also often advantageous to record a process gas used in the mass spectrometer (carrier), for example argon or other noble gases. For many applications, especially GD-MS, it is advantageous to lower the detection limit for impurities or doping. Minute traces of the components which are present should be detectable, if possible in the sub-ppb range, at the same time as the primary component (matrix). It is moreover desirable to take measurements efficiently and rapidly since, in GD-MS applications for example, analyte material is continually being eroded from the sample surface. The material composition may vary as a function of the depth of the sample.